Human erythropoietin is a haematopoietic cytokine required for the differentiation and proliferation of precursor cells into red blood cells. It activates cells by binding and orientating two cell-surface erythropoietin receptors (EPORs) which trigger an intracellular phosphorylation cascade. The half-maximal response in a cellular proliferation assay is evoked at an erythropoietin concentration of 10 pM, 10(-2) of its Kd value for erythropoietin-EPOR binding site 1 (Kd approximately equal to nM), and 10(-5) of the Kd for erythropoietin-EPOR binding site 2 (Kd approximately equal to 1 microM). Overall half-maximal binding (IC50) of cell-surface receptors is produced with approximately 0.18 nM erythropoietin, indicating that only approximately 6% of the receptors would be bound in the presence of 10 pM erythropoietin. Other effective erythropoietin-mimetic ligands that dimerize receptors can evoke the same cellular responses but much less efficiently, requiring concentrations close to their Kd values (approximately 0.1 microM). The crystal structure of erythropoietin complexed to the extracellular ligand-binding domains of the erythropoietin receptor, determined at 1.9 A from two crystal forms, shows that erythropoietin imposes a unique 120 degrees angular relationship and orientation that is responsible for optimal signalling through intracellular kinase pathways.

The crystal structure of the human major histocompatibility complex class I B allele HLA B*3501 complexed with the 8-mer peptide epitope HIV1 Nef 75-82 (VPLRPMTY) has been determined at 2.0 angstrom resolution. Comparison with the crystal structure of the closely related allele HLA B*5301 reveals the structural basis for the tyrosine specificity of the B*3501 F pocket. The structure also reveals a novel conformation of the 8-mer peptide within the binding groove. The positions of the peptide N and C termini are nonstandard, but the classic pattern of hydrogen bonding to nonpolymorphic MHC class I residues is maintained, at the N terminus by addition of a water molecule, and at the C terminus by a substantial shift in the alpha 2 helix.

The structure of the human MHC class I molecule HLA-B53 complexed to two nonameric peptide epitopes (from the malaria parasite P. falciparum and the HIV2 gag protein) has been determined by X-ray crystallography at 2.3 angstrom resolution. The structures reveal the architecture of a Pro-specific B pocket common to many HLA-B alleles. Relative to other alleles, the B53 peptide-binding groove is widened by a significant (up to 1.25 angstrom) shift in the position of the alpha 1 helix. Within this groove, bound water molecules, acting in concert with the side chains of polymorphic residues, provide the functional malleability of the MHC, which enables the high affinity/low specificity binding of multiple peptide epitopes.

In the cellular immune response, recognition by CTL-TCRs of viral antigens presented as peptides by HLA class I molecules, triggers destruction of the virally infected cell (Townsend, A.R.M., J. Rothbard, F.M. Gotch, G. Bahadur, D. Wraith, and A.J. McMichael. 1986. Cell. 44:959–968). Altered peptide ligands (APLs) which antagonise CTL recognition of infected cells have been reported (Jameson, S.C., F.R. Carbone, and M.J. Bevan. 1993. J. Exp. Med. 177:1541–1550). In one example, lysis of antigen presenting cells by CTLs in response to recognition of an HLA B8–restricted HIV-1 P17 (aa 24–31) epitope can be inhibited by naturally occurring variants of this peptide, which act as TCR antagonists (Klenerman, P., S. Rowland Jones, S. McAdam, J. Edwards, S. Daenke, D. Lalloo, B. Koppe, W. Rosenberg, D. Boyd, A. Edwards, P. Giangrande, R.E. Phillips, and A. McMichael. 1994. Nature (Lond.). 369:403– 407). We have characterised two CTL clones and a CTL line whose interactions with these variants of P17 (aa 24–31) exhibit a variety of responses. We have examined the high resolution crystal structures of four of these APLs in complex with HLA B8 to determine alterations in the shape, chemistry, and local flexibility of the TCR binding surface. The variant peptides cause changes in the recognition surface by three mechanisms: changes contributed directly by the peptide, effects transmitted to the exposed peptide surface, and induced effects on the exposed framework of the peptide binding groove. While the first two mechanisms frequently lead to antagonism, the third has more profound effects on TCR recognition.

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This study demonstrates that use of structural information improves the definition and optimization of cytotoxic T lymphocyte (CTL) epitopes. Epitope optimization usually requires numerous truncated peptides or a reverse immunogenetic approach, where the peptide binding motif is used to predict epitopes. These binding motifs do not reliably predict all peptides which are CTL epitopes. Comparison of 24 peptides eluted from HLA-B8 with 10 HLA-B8-restricted defined CTL epitopes demonstrated that known epitopes varied considerably at anchor positions. We used structural information based on determination of the crystal structure of the HLA-B8-GGKKKYKL complex to reassess previously described CTL epitopes, to predict new epitopes, and to predict the consequences of naturally occurring variation within epitopes. These predictions were confirmed by cytotoxicity and binding assays. Use of combined structural and immunological data more accurately defines the true peptide-binding motif of a restriction element than eluted peptide data allows.

Recombinant HLA-A2, HLA-B8, or HLA-B53 heavy chain produced in Escherichia coli was combined with recombinant β2-microglobulin (β2m) and a pool of randomly synthesised nonamer peptides. This mixture was allowed to refold to form stable major histocompatability complex (MHC) class I complexes, which were then purified by gel filtration chromatography. The peptides bound to the MHC class I molecules were subsequently eluted and sequenced as a pool. Peptide binding motifs for these three MHC class I molecules were derived and compared with previously described motifs derived from analysis of naturally processed peptides eluted from the surface of cells. This comparison indicated that the peptides bound by the recombinant MHC class I molecules showed a similar motif to naturally processed and presented peptides, with the exception of the peptide COOH terminus. Whereas the motifs derived from naturally processed peptides eluted from HLA-A2 and HLA-B8 indicated a strong preference for hydrophobic amino acids at the COOH terminus, this preference was not observed in our studies. We propose that this difference reflects the effects of processing or transport on the peptide repertoire available for binding to MHC class I molecules in vivo.

Recent crystallographic results have provided close to atomic resolution views of the recognition events mediated by MHC class I molecules. The specificity-conferring interaction of MHC class I/peptide with a T-cell antigen receptor (TCR) appears dependent on certain key interactions with the MHC scaffold. These interactions, in particular those of the TCR V alpha domain, define a standard orientation for TCR binding. Previous studies on biologically significant variations in the TCR recognition surface presented by a series of MHC/variant peptide complexes can be reassessed in the light of this TCR-binding mode. The interaction of CD8 with MHC class I resembles that between antibody and antigen in the use of loops from the CD8 structure. The interaction is of very low affinity and buries equivalent surface area to that between the TCR and MHC class I but while the TCR/MHC interface shows poor surface shape complementarity the match in the conservative interaction between MHC and CD8 is precise.